The subject matter disclosed in this application relates to a system and method for protecting an extended passive optical network (PON).
Network operators deliver Internet, television and telephone services to consumers using fiber-to-the-premises (FTTP) architectures. Many of these deployments have used PON, rather than point-to-point access networks.
Referring to FIG. 1 of the drawings, an Optical Line Termination (OLT) 10 in a telephone company central office connects to service platforms, such as Internet broadband network gateways (BBNG), Internet Protocol Television servers, and Voice over IP gateways, typically via a metro network 12, and a PON connects the OLT 10 to Optical Network Terminations (ONTs) 14. In typical single family dwelling units, the ONT is located in or attached to the exterior of the home. In typical multifamily dwelling units, the ONT is located in a common utility space or located inside individual living units. In either case, the ONT derives service interfaces, such as Ethernet, cable television and analog phone, from the signal on the PON.
The PON comprises a feeder fiber 16 that connects the OLT to a passive remote node that includes an optical power splitter 18. Typically, the splitter has a split ratio of 16:1, 32:1 or perhaps 64:1, depending on the optical power budget of the network. The fan-out ports of the splitter are connected to distribution fibers 22, each of which is further connected to an ONT 14 via a drop fiber (not separately shown).
Under current industry practice, such a network may utilize an optical carrier at 1490 nm for downstream communication (i.e. transmission of bitstreams from the OLT to the customers' ONTs) and may utilize an optical carrier at 1310 nm for upstream communication (i.e. transmission of bitstreams from the customers' ONTs to the OLT).
The total reach of the PON, i.e., the maximum of the sum of the length of the feeder, distribution and drop fibers, is determined by the optical power budget of the system and the split ratio of the optical splitter. For example, ITU-T Recommendation G.984.2amd1 specifies an optical power budget of 28 dB. This equates to 20 km reach with a 32:1 split ratio, or 10 km reach with a 64:1 split ratio. While 20 km reach is adequate for many deployments, a network operator often needs a longer reach. For example, homes in rural areas might not be within 20 km of a central office. Further, a network operator may wish to reduce the number of central offices in its network so as to eliminate cost.
As shown schematically in FIG. 2, the effective reach of a PON may be extended by connecting the OLT to the upstream end of the feeder fiber 16 through a backhaul fiber 30 and an extender 26 including gain elements 32 which produce optical gain and thereby increase the optical power budget of the PON. The upstream and downstream signals are separated by wavelength division multiplexers 34. One or both gain elements may be implemented by a semiconductor optical amplifier (SOA). An SOA is enabled by applying bias current to the SOA, in which case the SOA exhibits gain; it is disabled by removing bias current from the SOA, in which case the SOA exhibits a high extinction ratio. Alternatively, one or both gain elements may be a doped fiber amplifier, such as an erbium doped fiber amplifier, which may be enabled by applying bias current to the pump laser, in which case the amplifier exhibits gain; it may be disabled by removing bias current from the pump laser, in which case the doped fiber absorbs light. A third possibility is to implement one or both gain elements with an optical to electrical to optical (OEO) regenerator, which recovers the optical signal, converts it to electrical form, possibly recovers and regenerates timing, converts the electrical signal back to optical form, and transmits the regenerated optical signal. An OEO may be enabled by completing the transmit path in the regenerator, including its receiver, clock/data recovery, buffering and transmitter, and may be disabled by turning off any part of the transmit path (but most conveniently the transmit laser).
The resiliency of an extended PON is a concern for network operators. Vulnerability of fiber to breakage, e.g., due to accidental dig-ups, is roughly proportional to its length, and an extended PON by definition has longer fiber sections than a standard PON. Further, an amplified PON may serve more subscribers than a standard PON or a regenerator extended PON and these additional subscribers constitute a larger shared risk group. Further, recent events have heightened sensitivity to the time required to restore service in the event of loss of a central office, e.g., due to a flood, fire or act of terrorism; at the same time, if a network operator attempts to reduce the number of central offices in its network, a larger number of subscribers will be served from each central office. Thus, it is desirable to provide a protection scheme that will protect against failure of at least the feeder fiber, the backhaul fiber, the extender, and the OLT.
A protected extended PON may comprise a plurality of ONTs, with drop and distribution fibers, and a remote node, as in a standard PON; and a working entity and a protection entity, wherein each entity of the working and protection pair comprises an OLT, a backhaul fiber, an extender unit, and a feeder fiber. The two feeder fibers may be diversely routed; the two backhaul fibers may be diversely routed; and the two OLTs may be located in the same central office or in different central offices.
The two feeder fibers feed respective fan-in ports of a 2:N optical power splitter. Advantageously, optical power is distributed equally in the downstream direction from each fan-in port, and equally in the upstream direction to each fan-in port; thus, there is no optical power penalty to, e.g., a 2:64 power splitter relative to a 1:64 power splitter
However, a problem arises with such a topology. The 2:N splitter is an entirely passive device, and thus signals from both OLTs pass through it to the ONTs. If there were two such signals, they would mutually interfere. Thus, it is necessary to ensure that downstream signals from only one OLT at a time reach the splitter. This may be accomplished by ensuring that only one OLT of the working and protection pair is enabled at a time and either disabling or suppressing the downstream signal of the other OLT. The enabled and disabled states must be reversed in the event that a fault is detected, but only if the protection entity is intact.
ITU-T Recommendation G.983.5 describes several protection schemes for unextended PONS. Type B protection protects the feeder and OLT line terminations, but not the ONTs. Type C protection also protects ONTs, and thus solves a different problem. Notably, working and protection OLT line terminations must be in the same chassis. This means that no protection is provided in the event of loss of a central office.
Past approaches to the problem of protecting an unextended PON have used line terminations within a single OLT chassis and have been coordinated by local mechanisms within the chassis. Extending the problem to OLTs which are not collocated raises the problem of coordination between potentially distant chassis. An extended PON creates the possibility of various failure modes, which could lead to false detection of a fault or failure to detect a fault in a working PON. For example, if the protection OLT were to simply perform protection switching when it detected a loss of signal from the PON, a fault in the upstream path of the protection extender would result in a false detection. Similarly, if the protection OLT were to depend upon a failure signal from the working OLT to determine a failure, then a fault in the communication path between the two OLTs would result in either failure to detect a subsequent fault in the PON, or immediate false detection. A person sufficiently skilled in the art of protocol design may be able to identify and devise ways to remedy these problems but the resulting mechanism may be complex and unwieldy in ways that may create exposure to implementation defects.
If fault detection is located in the OLT, faults that affect one direction of transmission in the extender cannot be detected.